What rpm stall converter should use?

PG Duster said
I’m in a bad place without knowing the camshaft specs or the cylinder head valve size? The engine was built by the person I bought the car from.
What makes 904 a better transmission than a 727?

See if you can’t get in touch with the last owner of the car for the cam specs. This would be the #1 important item to know. Valve size is not a big deal deal. The 904 is better because it is lighter in overall weight and requires less power to make it work. The difference is a tenth or better in a race car. Swapping the 904 to the 727 will require your driveshaft to be shortened & use a different yoke.

PG Duster said
I bought the car in February of this year, the engine was already rebuilt, the person said it’s a mild cam, which I can hear, and after market steel cylinder heads. The person was supposed to send me the specs, But you know how that goes I don’t know what the numbers are, he never sent them.

Try and get a hold of him again. Be a pest!
Until he says Uncle.
Or swap in your own cam ;)